The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, to a method for reconstituting a fuel assembly by removal and attachment of top nozzles and to a reconstituted fuel assembly with an improved top nozzle attachment system.
Conventional designs of fuel assemblies include a multiplicity of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length. The grids are attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles which extend slightly above and below the ends of the fuel rods. At the top end of the assembly, the guide thimbles are attached in openings provided in the top nozzle. The guide thimbles may each be surrounded by a sleeve for attachment to the top nozzle and to the uppermost grid.
During operation of such assembly in a nuclear reactor, the fuel rods may occasionally develop cracks along their length resulting primarily from internal stresses thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored.
Under these circumstances, it is difficult to detect and remove failed fuel rods because they are part of an integral assembly of guide tubes welded to the top and bottom nozzles. Consequently, to gain top access to individual fuel rods, it is necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the top nozzle to the control rod guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in a reactor because of the damage done to both the guide thimbles and the top nozzle which prohibits rewelding.
In view of the high costs associated with replacing fuel assemblies, both domestic and foreign utilities have indicated an interest in reconstitutable fuel assemblies in order to minimize both their operating and maintenance expenses.
Conventional reconstitutable fuel assemblies incorporate design features (at the time of original manufacture) arranged to permit the rapid removal of individual failed fuel rods, the option to replace rods, followed by the additional use in the reactor and/or normal handling and storage of the affected fuel assembly. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble assembly, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in neutron-absorbing liquid. With rod removal/replacement and after the top nozzle has been remounted on the control rod guide thimble tubes, the reconstituted assembly can then be reinserted into the reactor and used until the end of its useful life, and/or stored in spent fuel pools or other places in a safe, normal manner.
The conventional design philosophy for existing reconstitutable fuel assemblies has been to implement their costly reconstruction features on every fuel assembly at the time of original manufacture. The various threaded or other arrangements to removably attach each control rod guide thimble to the top nozzle represent a high cost in time and money for the added manufacturing operations, when only a very small percentage of the fuel assemblies will ever experience a leaky fuel rod and hence the need for reconstitution.